1. Field of the Invention
The present invention relates to a zoom lens barrel, or more particularly, to a mechanism for a zoom lens barrel capable of continuously varying the focal length of a photographic optical system.
2. Description of the Related Art
In the past, a variable-power lens (hereinafter zoom lens) capable of continuously varying the focal length of a photographic optical system, which realizes the variable-power lens, has generally been employed in photographic apparatuses in practice. The photographic apparatuses include a compact camera for carrying out photography using a silver film, and an electronic still camera (hereinafter electronic camera) for recording a picture signal or picture information output from an imaging means such as a CCD (hereinafter these types of cameras are generically called cameras). In recent years, there has been an increasing demand for a zoom lens offering a higher power. The overall length of a lens barrel for holding a photographic optical system realizing the zoom lens (photographic lenses) tends to get longer with an increase in power or focal length.
In this case, when the overall length of the lens barrel increases, portability that is a great merit of general compact cameras will be impaired terribly. For this reason, a so-called collapsible zoom lens barrel has been widely adopted and has generally prevailed. The collapsible zoom lens barrel is such that: when a camera is unused or carried, the lens barrel is moved inside a camera body so that it will jut out of the camera body by the smallest possible magnitude.
On the other hand, talking of the camera, an automated camera having an automatic focus (AF) unit or automatic exposure (AE) control unit for autonomously adjusting a focal point or exposure value has widely prevailed. Such an automated camera realizes automatic focus (AF) or automatic exposure control (AE).
In the case of a collapsible zoom lens barrel adopted for such an automated camera, a group of photographic lenses constituting a photographic optical system is composed of two to four groups of lenses. The plurality of groups of photographic lenses is borne by lens frames provided respectively for the groups of photographic lenses. The zoom lens barrel is therefore composed of the same number of lens frames as the number of groups of photographic lenses.
The plurality of lens frames is moved in the direction of an optical axis of photographic lenses by a given distance by means of a cam cylinder that rotates with the optical axis as a center. Movements to be made by the lens frames at this time are a movement within a power varying (zooming) zone in which power variation is carried out, and a movement within a collapsing zone in which collapse is carried out. The cam cylinder is designed to move over the two different zones.
As for the cam cylinder, a form composed of two separate cam cylinders of a zooming cam cylinder for realizing a power varying zone and a collapsing cam cylinder for realizing a collapsing zone has been adopted in the past. Moreover, there is a trend toward a compact camera body. Therefore, a form in which a cam realizing the power varying zone and a cam realizing the collapsing zone are formed continuously in one cam cylinder has often been adopted these days.
On the other hand, there is a macro zoom lens barrel offering a nearby photography function (macro zooming function). A generally adopted macro zoom lens barrel is such that a cam realizing a power varying zone and a cam realizing a macro zooming zone are formed continuously in one cam cylinder. This lens barrel enables a lens frame to move over both zones.
As mentioned above, in a zoom lens barrel that has been used generally in practice, two cam grooves delineated by different cam curves are cut continuously in one cam cylinder. The cam cylinder is turned in order to induce two movements.
Using this type of zoom lens barrel, when the cylindrical cam is turned, a desired lens frame is moved by the cam realizing a power varying zone. The desired lens frame is thus moved for varying the power of the photographic lenses. Moreover, the lens frame may be moved continuously for causing the zoom lens barrel to collapse or make any other movement. A moving means for allowing the zoom lens barrel to collapse or make movements other than the movements for varying the power can be excluded. This brings about the merit that the zoom lens barrel and a camera employing the zoom lens barrel can be designed to be compact and lightweight and to cost least.
FIG. 10 is a diagram showing the development of a cam cylinder included in a collapsible zoom lens barrel to which the foregoing form is adapted. Movements to be made by a photographic optical system that is moved by the cam cylinder will be described below. The development of FIG. 10 shows the cam cylinder seen from the inner circumferential side thereof.
FIG. 9A, FIG. 9B, and FIG. 9C schematically show the positions of groups of photographic lenses associated with the states of a conventional zoom lens barrel. FIG. 9A shows a collapsed state in which the groups of photographic lenses are located at stored positions in a non-photographic state. FIG. 9B shows a state in which the groups of photographic lenses are located at short-focus (or wide-angle) positions in a photographic state. FIG. 9C shows a state in which the groups of photographic lenses are located at long-focus (or telephotographic) positions in the photographic state and. To begin with, the positional relationship among the groups of photographic lenses contained in the zoom lens barrel will be described in relation to the states of the groups of photographic lenses shown in FIG. 9A, FIG. 9B, and FIG. 9C.
The photographic optical system contained in the zoom lens barrel is composed of four groups of photographic lenses. The groups of photographic lenses are arranged in order of a first group of lenses 11, a second group of lenses 21, a third group of lenses 31, and a fourth group of lenses 41, moving from the side of an object 101 towards the camera. The first to fourth groups of lenses 11, 21, 31, and 41 are borne separately by four lens frames. The four lens frames are designed to move to given positions with the turn of the cam cylinder that will be described later (See FIG. 10). In FIG. 9A, FIG. 9B, and FIG. 9c, the four lens frames and cam cylinder 160 are not shown for fear of complication of the drawing.
To begin with, assume that the zoom lens barrel is placed in the collapsed state (stored position) as shown in FIG. 9A. The main power supply of a camera in which the lens barrel is incorporated is turned on and the camera is shifted to a photographic mode. The cam cylinder 160 (See FIG. 10) of the lens barrel of the camera is then turned, whereby the groups of photographic lenses are moved to enter a ready-to-photograph state shown in FIG. 9B (short-focus positions in the photographic state). At this time, only the first group of lenses 11 and second group of lenses 21 are moved but the third group of lenses 31 and fourth group of lenses 41 are not moved.
In the photographic state, when a movement is made to carry out power variation (zooming), the groups of photographic lenses are moved between the short-focus positions shown in FIG. 9B and the long-focus positions shown in FIG. 9C. However, when the movement is made to carry out power variation, the first group of lenses 11 is not moved.
In the photographic state, the main power supply of the camera is turned off in order to terminate photographic movements. This causes the groups of photographic lenses to move to the stored positions shown in FIG. 9A by way of the short-focus positions shown in FIG. 9B. The groups of photographic lenses are thus placed in the collapsed state.
Thus, the groups of photographic lenses are moved owing to the operation of the cam cylinder 160 (See FIG. 10).
Next, the operation of the cam cylinder 160 will be described in conjunction with FIG. 10.
The cam cylinder 160 causing the lens frames to move is realized with a hollowed cylinder. A plurality of cam grooves is formed in the outer circumference or inner circumference of the cylinder. The lens frames are arranged at given positions on the inner and outer circumferences of the cam cylinder 160 so that the lens frames can slide freely. Given cam pins are planted on the outer or inner circumferences of the lens frames. The given cam pins are engaged with the given cam grooves in the cam cylinder 160.
FIG. 10 shows, as mentioned above, the development of the cam cylinder 160 in the collapsible zoom lens barrel. The plurality of cam grooves formed in the circumferences of the cam cylinder 160 is illustrated. 0.degree., 90.degree., 180.degree., 270.degree., and 360.degree. in FIG. 10 means the angles of rotation of the cam cylinder 160. Moreover, the cam cylinder 160 is selectively turned in the directions of arrows X in FIG. 10. The lens frames are moved in the direction of an optical axis (directions of arrows Y). Herein, the direction of arrow Y1 is a direction towards an object.
The cam cylinder 160 has a first cam groove 161 (indicated with a dashed line), which causes the first lens frame to move, formed in the outer circumference thereof. A second cam groove 162, third cam groove 163, and fourth cam groove 164 are formed in the inner circumference thereof.
The detailed shapes of the thus formed cam grooves in the zoom lens barrel will be described by taking the second cam groove 162 for instance.
The second cam groove 162 is realized by continuously forming grooves in a power varying zone cam Zu and a collapsing zone cam Co. A cam curve delineating the second cam groove 162 is coincident with the lens frame, and has the same angle as the angle of rotation of the cam cylinder 160.
A terminal end C2 of the groove in the collapsing zone cam Co is equivalent to the stored position in the collapsed state shown in FIG. 9A. Moreover, a full wide-angle position W2 that is the position of one end of the groove in the power varying zone cam Zu is equivalent to the short-focus position shown in FIG. 9B. The other end T2 thereof is equivalent to the long-focus position shown in FIG. 9C. A zone between the short-focus and long-focus positions is a photographic zone.
The second cam groove 162 is realized with a groove having a given width with a cam curve as a center. A second cam pin 122 of the second lens frame is engaged with the groove 162. FIG. 10 shows a photographic state in which the second cam pin 122 is located at the short-focus position (full wide-angle position) W2 that is the position of one end of the groove in the power varying zone cam Zu. The same applies to the relationship between each cam groove and each cam pin, that is, between the first cam groove 161 and a first cam pin 112, between the third cam groove 163 and a third cam pin 132, and between the fourth cam groove 164 and a fourth cam pin 142.
Moreover, the lens frames are partly engaged with two guide shafts arranged parallel to the optical axis of the photographic lenses so that the lens frames can slide freely. The lens frames can thus be supported to be able to move only in the direction of the optical axis. The cam cylinder 160 is turned by a motor (not shown) designed to drive the lenses arranged in the camera body or lens barrel. The lens frames (groups of photographic lenses) are thus moved in the direction of the optical axis by the cam pins engaged with the cam grooves. Given positions to which the lens frames are moved are determined by the cam curves delineating the cam grooves.
As mentioned above, the zoom lens barrel has been used in the camera in the past. In the zoom lens barrel, the cam pins planted on the lens frames for separately holding the plurality of groups of photographic lens are engaged with the plurality of cam grooves formed in the cam cylinder. When the cam cylinder is turned by the motor, the zoom lens barrel is moved to collapse or to vary the power of the photographic lenses.
The cam cylinder has been created in the past by machining a metallic member. In recent years, the cam cylinder has been manufactured by performing, for example, injection molding using a member made of a reinforced plastic or the like. This is intended mainly to reduce the cost of manufacturing or realize a lightweight design.
As mentioned above, one cam cylinder may be used to manufacture a zoom lens barrel caused to make two different movements. For example, the zoom lens barrel is moved to collapse and moved for varying the power, or the zoom lens barrel is moved for varying the power and moved for carrying out macro zooming. In this case, injection molding may be adopted for manufacturing a cam cylinder that has a cam groove made by continuously forming grooves in a collapsing zone cam and power varying zone cam or in the power varying zone cam and a macro zooming zone cam. This poses problems, as described below.
A first problem lies in that since the cam groove in the cam cylinder is formed by employing a sliding die assembly, the opposed walls of the cam groove may not be parallel to each other. The opposed walls may be inclined mutually and the cam groove may be tapered. The cam pin to be engaged with the cam groove is accordingly tapered. However, since the cam groove is delineated with two cam curves for a power varying zone and collapsing zone respectively, the cone angle of the cam pin becomes unnecessarily large. The first problem is attributable to this fact.
The problem that when an attempt is made to form a cam groove by performing normal injection molding, the cam groove is tapered will be described below.
FIG. 11 is a side view of a cam cylinder having a typical shape, that is, having a cam groove formed in the outer circumference of a cylindrical member. FIG. 12, FIG. 13, and FIG. 14 show a cross section of the cam cylinder along a 12--12 line in FIG. 11. FIG. 12 shows the shape of a cam groove cut by machining a metallic cam cylinder as conventionally performed. FIG. 13 and FIG. 14 show the shapes of cam grooves cut in a cam cylinder, which is an injection mold, using a sliding die assembly as conventionally performed.
Referring to FIG. 11, the shape of a groove portion 168 equivalent to part of a cam groove is compared with the shape of a groove portion 169 equivalent to another part thereof. The groove portion 169 is separated by an angle from the groove portion 168. When the cam groove is created by machining a cylindrical member as conventionally performed, the groove portion 168 and groove portion 169 are both, as shown in FIG. 12, formed in radial directions with respect to a center axis of the cam cylinder 160. As for the sectional shape, the opposed inner walls of each of the groove portions are mutually parallel. A cam pin can therefore meet the walls of the cam groove at the same angle all over the cam groove. Moreover, the cam pin should merely be shaped cylindrically. Thus, the cam pin can be engaged with the cam groove without a problem.
By contrast, as far as an injection mold that has been generally employed in recent years is concerned, the cylindrical part of the cam cylinder 160 is molded by performing core molding. In the core molding, assembled cores are moved in the direction of the optical axis. A cam groove is usually cut using a sliding die assembly except for the cam grooves, which are arranged at intervals of a certain pitch, in a special cam that can be rotationally molded. Moreover, the special cam can be removed while being turned.
In this case, the sliding die assembly may be moved in a so-called direction of outward sliding (a direction of arrow X in FIG. 12). A radial direction of the cam cylinder 160 is parallel to the direction in which the sliding die assembly is moved. The groove portion 168 formed in this radial direction has, as shown in FIG. 13, the same shape as that in FIG. 12.
By contrast, another radial direction is not parallel to the direction of the sliding die assembly. The groove portion 169 formed in this direction has one side thereof so-called undercut. For coping with undercutting, the groove portion is reshaped into a groove portion 169a. The groove portion 169a has a sectional shape whose one side is, as shown in FIG. 13, inclined relative to the radial direction and made parallel to the direction of sliding. The sectional shape of the groove portion is quite uncertain. An injection mold is therefore normally subjected to the means described below.
As shown in FIG. 14, a groove portion 168a is shaped in consideration of the shape of a cam pin to be engaged with the groove portion. Specifically, both sides of the groove portion have a given inclination. A groove portion 168a is also shaped so that both sides thereof will have a given inclination.
In line with a cam groove having a given inclination, a cam pin is tapered as described below. FIG. 15 is an enlarged view showing a major portion of the cam groove cut in the cam cylinder shown in FIG. 11, thus illustrating cross sections along CC, DD, and EE lines. The cam groove in FIG. 15 is equivalent to the second cam groove 162 in FIG. 10.
In FIG. 15, there is shown a power varying zone cam Zu. The power varying zone cam Zu is a cam having a groove that causes a lens frame to move by a small distance that is small for an angle of rotation of a cam cylinder. An angle .theta.2 between a cam curve F and a plane G orthogonal to the optical axis is small (the cam curve is "shallow").
Moreover, a collapsing zone cam Co is, unlike the power varying zone cam Zu, a cam having a groove that causes the lens frame to move by a large distance that is large for the angle of rotation of the cam cylinder. An angle .theta.1 between the cam curve F and the plane G orthogonal to the optical axis is larger than the angle .theta.2 (the cam curve is "deep").
To begin with, a cam groove cut using a sliding die assembly will be discussed. An inclination .theta.3 of an EE section is the same between the grooves in the power varying zone and collapsing zone. The cam curve F is inclined relative to the optical axis. An actual angle of opening of a groove portion, which meets a cam pin, on a section perpendicular to the cam curve F falls into angles .theta.4 and .theta.5 (See CC and DD sections in FIG. 15).
The larger the angles .theta.1 and .theta.2 between the cam curve F and the plane perpendicular to the optical axis are, the larger the angles of opening .theta.4 and .theta.5 are. The angle of opening in the power varying zone cam delineated by the shallow portion of the cam curve F, .theta.4, is smaller than the angle of opening in the collapsing zone cam delineated by the deep portion of the cam curve F, .theta.5. Incidentally, when the cam curve F is parallel to the cam cylinder (optical axis), the angle of opening should be 0.
However, in reality, a lens frame is provided with only one cam pin to be engaged with both of the grooves in the cams. The cone angle of the cam pin cannot be set to two values matching the angles of opening of the grooves. The cone angle of the cam pin is therefore set to match a larger one of the angles of opening. In the example shown in FIG. 15, both of the grooves are shaped to have the angle of opening .theta.5. Accordingly, the cone angle of the cam pin is set to a value equivalent to the angle of opening .theta.5.
Consequently, the power varying zone cam whose groove can be shaped to have a smaller angle of opening in principle must be tapered at an unnecessarily large inclination. The cone angle of the cam pin is made larger accordingly. This poses problems described below.
1) As the cone angle of the cam pin gets larger, a magnitude of misfit between the cam pin and cam groove stemming from a backlash in a radial direction tends to increase. Consequently, positioning precision deteriorates.
In other words, a slight gap preserved in a radial direction between the lens frame and cam cylinder for the purpose of smoother movements gets larger proportionally to the cone angle of the cam pin. Consequently, a mismatch of the lens frame occurring in the direction of positioning increases.
The mismatch of the lens frame occurring in the direction of positioning has posed a problem in the past. For solving the problem, for example, Japanese Unexamined Patent Publication No. 62-112109 has disclosed various technological means. A zoom lens that is disclosed in the Japanese Unexamined Patent Publication No. 62-112109 has solved the problem that the mismatch of the lens frame occurs in the direction of positioning. Specifically, a substantially arc-shaped constraining spring is laid along the outer circumferences of the lenses, and a cam pin is pushed against a wall of a cam groove.
2) When the cone angle of a cam pin is large, there arises a problem that the cam pin is likely to come off a cam groove or, on the contrary, to bite into the cam groove. An extraneous force may be applied especially to a first lens frame located foremost (nearest an object) in a lens barrel. The problem is most likely to occur in the first lens frame.
Moreover, for designing a compact and lightweight lens barrel, the diameter of the lens barrel may be decreased or a length in a radial direction by which a lens frame is engaged with the lens barrel may be decreased. Otherwise, the thickness of a cam cylinder may be decreased. In this case, the foregoing problem is more likely to occur.
In the example shown in FIG. 15, the sliding die assembly is of an outward sliding type that is moved outward of a cam cylinder. Alternatively, the sliding die assembly may be of a type being moved inward. Nevertheless, the same can apply to the type of sliding die assembly. However, the direction of opening is reversed.
Next, a second problem occurring when a cam cylinder having a cam groove composed of two grooves in different cams is manufactured by performing injection molding will be described. The second problem is deterioration of precision in dimensions of a cam groove.
In the aforesaid prior art, as shown in FIG. 10, an extreme telephotographic position T1 in the first cam groove 161 and an extreme wide-angle position W2 in the second cam groove 162 are located on the outer and inner circumferences of the cam cylinder. Likewise, a stored position C1 in the first cam groove 161 and an extreme telephotographic position T3 in the third cam groove 163 are located on the outer and inner circumferences of the cam cylinder. The extreme telephotographic position T1 and extreme wide-angle position W2, and the stored position C1 and extreme telephotographic position T3 have a positional relationship that they are nearly overlapping. In the areas in which the positions are overlapping, the thickness of the cam cylinder varies locally greatly. This results in a so-called drop. The dimensions of a cam groove slightly differ because of the drop.
In general, when precision must be ensured for cam grooves, the cam grooves must be arranged in a cam cylinder in full consideration of the restrictions placed due to molding. However, as mentioned above, the power varying zone cam and collapsing zone cam are formed continuously, and grooves cut in the cams constitute one large cam groove. The layout of cam grooves in the surface of the cam cylinder is restricted by the relationship among the cam grooves causing lens frames to move. It becomes very hard to attain both the compactness of the cam cylinder and the precision in dimensions of the cam grooves.
Moreover, when the cam grooves overlap while lying on both sides of the cam cylinder, the thickness of the cam cylinder gets small in that area. If a load of a drop or any other impact were imposed on the cam cylinder, the load would be concentrated on the thin area. Consequently, the cam cylinder would be deformed.